Accounting profit is necessary for publicly traded companies to survive; it's not a sign of extortion.

Profit is seen as a pretty ugly thing for public services to be dealing in. Take the Guardian's Terry Macalister in April (only picked because it's the most recent I can find):

The big six energy suppliers have been accused of "cold-blooded profiteering" after official figures showed they had more than doubled their retail profit margins over the last 18 months and were now earning an average of £95 profit per household on dual-fuel bills.

To be clear, the profit motive is a fair target. There's a real debate to be had over whether or not companies providing public services should be operating under a legal structure which requires them to try to maximise the amount of cash (over the long term) they can return to shareholders, rather than, say, maximising the quality of service provided for a given investment, or providing a set level of service at the minimum cost possible.

But given public services are frequently run by private companies, attacking the amount of profit they actually make is concerning, for one simple reason: money costs money.

It's a basic fact of the economy, one which explains why it takes so long to pay off credit card bills, why the bank pays you if you've got a savings account, and why Greece is finding things tricky at the moment.

But while we're all familiar with debt finance – the act of borrowing a sum, and then paying it back with interest – corporations have an alternative way of paying for the money they need: equity finance. Rather than paying interest on top of borrowed cash, they return a share of the money they make with their loans to the people who loaned to them in the first place.

That money being returned – the equivalent of the interest which we all have experience paying – is profit.

If companies don't earn some profit, then the shareholders are likely to cash out, safe in the knowledge that they can earn more by putting their money elsewhere – maybe by buying shares in another company, or putting it in a high interest savings account. The amount of profit that companies have to earn to stop this happening will vary based on the perceived riskiness of investing in them, as well as the value of investments elsewhere, and is known as the "cost of capital".

Power companies need to be able to make investments, frequently valued in the billions of pounds (Macalister quotes one industry analyst who estimates £50bn is needed just to hook up new gas supplies). It's only by making profit today – that is, by rewarding the shareholders who bought in to the companies before – that they can ensure that they have enough funding to carry on paying for investments tomorrow.

None of this is to say that there can't be such a thing as "too much" profit; if Thames Water were to suddenly make Apple-sized margins, we could be pretty sure that they were overcharging or underinvesting. But simply making accounting profit, even at the same time as pleading penury and raising prices, is not a sign of underhandedness. It's just a sign of a business working as normal.

Companies which deliberately and continually make no profit do exist. But they aren't traded on the open market, and have no access to equity finance. That's fine for some, but worrisome if they suddenly need to find large amounts of cash to invest – or to stave off the creditors.

Perhaps public services should be run as non-profits, or not be run privately at all; but if they are, attacking them for making profit is foolish.